Electronic transformers

[Guest lightnix]

I've Googled around a little, but can't find any decent info, so...

 

How do electronic transformers work and how do they differ from "traditional" magnetic / wirewound ones?

 

The main reason I'm asking this is because I'd also like to know why LED lamps won't run from them. No damage occurs - there's just a brief flicker and then nothing :(

 

Any thoughts / clues / knowledge on this?

[Andrew C]

I too am a bit in the dark as to how they work, but from empirical experience they have a minimum load requirement. My guess it that they are some flavour of switch-mode psu.

[dbuckley]

Both sorts of "transformer" have a transformer in them, but the differ considerably in size and weight. A traditional transformer has a primary (mains) winding, one or more secondary windings. becuase the primary winding is connected to the mains, the frequency the trnasformer runs at is 50Hz.

 

An electronic transformer rectifies the mains to DC, and then "switches" that DC to the transformer primary at a freqeuncy generally between 10K and 40K times per second. They are a switched mode power supply.

 

The physics of transformers is that for a given load, the higher the frequency, the less iron the transformer has to contain, so the lighter (in weight!) and smaller the transformer.

 

Since with an electronic transformer the incoming AC is rectified, the only thing that matters is the peak voltage reached on the AC cycle, and for any dimmer setting allowing more than half the half-cycle through (which becuase of dimmer curves wont be 50% on the desk) the rectified DC voltage is equal to that of raw mains. As you dim below the half of half-cycle, the peak voltage drops, and usually eventually to a point where the electronics receives insufficient voltage to work, and then the transformer stops working and the load goes off. I'm sure some electronic transformers are cleverer than others, and measure what they are fed, so they can dim almost all the way to extinction reliably.

[Don Allen]

Some electronic transformers need a minimum load to operate as they have sense circuits to monitor the output voltage to help regulate the output. If you try to measure the electronic transformer output with a multimeter and no load, you may not get a reading as the electronic transformer is not oscillating. (you may also be trying to read a voltage outside the frequency range of your multimeter.)

 

In you case the the LED is too small a load and the brief flicker is the electronic transformer starting up then shutting down as it thinks there is a fault condition.

 

If you are replacing 50 watt halogen lamps with LED versions, some electronic transformers state you need a minimum of four led units per transformer which is roughly 25 watts with losses.